The present invention relates to a method for measuring the resistance of a load--preferably the firing pellet of an airbag--connected on the secondary side of a rotary transformer, by analyzing an electrical variable that can be sensed on the primary side of the transformer.
An arrangement is known from German Published Patent Application No. 38 12 633 A1 in which a rotary transformer is used, for example, for signal transmission between a control unit and an airbag built into a steering wheel of a vehicle. In order to continuously check the operability of an airbag so that it is immediately ready to operate in any emergency situation, the resistance of the firing pellet, which should lie in the range of, for example, 1.8.OMEGA. to 2.5.OMEGA., is subjected to a continuously repeating measurement. Transmission of signals by means of the inductively coupled windings of the rotary transformer depends on the spacing between the two windings, i.e. the size of the air gap between the dished cores of the windings. Installation and production tolerances of the steering wheel on which the secondary-side dished core of the rotary transformer is installed can lead to falsification of the measured resistance of the firing pellet. To prevent such measured-signal falsifications, the aforesaid existing art provides for an oscillator circuit on the secondary side of the rotary transformer that is excited from the primary side by a control signal. After the control signal is shut off, the decaying response signal of the oscillator circuit is transmitted back to the primary side, and the secondary-side resistance is determined from the time constant of the decaying response signal. With this measurement method, the resistance measured on the secondary side depends greatly on the radial and axial clearance between the primary and secondary windings of the rotary transformer. The accuracy with which the firing pellet resistance is measured is therefore not very high.